1. Field of the Invention
The present invention relates to the detection of gas in trace amounts. It more generally relates to a spectroscopy absorption measurement method in a resonant cavity.
2. Discussion of the Related Art
FIG. 1 illustrates the basic diagram of a method of spectroscopy absorption measurement in a resonant cavity. Light is emitted by a laser 1 in a resonant optical cavity 2 via an optical coupling system 3. The light coming out of the resonant cavity is received by a photodetector 4 and is sent to an analyzer 5. If the laser frequency varies, there will be a maximum of the signal received by the photodetector for each cavity mode. If the cavity contains a chemical species exhibiting an absorption line at the wavelengths of the injected photons, the transmission will be reduced according to the absorption. Based on the transmission spectrum, the absorption spectrum is obtained in the same way as for a conventional absorption spectrum. But, then, the absorption signal is multiplied by the cavity thinness and it should normally be possible to perform absorption measurements with a very high sensitivity. A transmission curve showing the transmitted intensity versus frequency expressed as a ratio of C/2L, where C is the speed of light and L is the length of the resonant cavity, is illustrated in FIG. 2A, and the absorption of the chemical species contained in the cavity versus frequency illustrated in FIG. 2B can be deduced from this curve.
Unfortunately, in practice, such methods of direct measurement by spectroscopy in a resonant cavity are impossible or very complex to implement. Indeed, a sufficient power has to be injected into the cavity, this power must be constant or follow a known variation, and the detected signal must not be too noisy.
To avoid these disadvantages, methods of detection by optical power decrease in a resonant cavity, currently designated as CRDS, for cavity ring-down spectroscopy have been used. According to this method, the laser beam is sent into the cavity, after which the photon injection is abruptly stopped. The photons then remain trapped in the cavity and their intensity decreases exponentially along time. If the cavity is empty, or for a wavelength that does not correspond to an absorption line of a gas contained in the cavity, this decrease will exhibit a given time constant essentially determined by the mirror losses at the considered wavelength. If the cavity contains a chemical species exhibiting an absorption line at the wavelength of the injected photons, this time constant will be reduced. An advantage of this method is that it gets rid of the noise due to fluctuations of the intensity injected into the cavity.
To inject a sufficient quantity of light into the resonant cavity, a method of CRDS type in which the frequency of a continuous semiconductor laser is controlled by an optical feedback from the cavity has been provided. Such a method is described in PCT patent application WO99/57542. This method provides satisfactory results, but the absorption curve determination time is relatively long since, for each measurement, a single point of the absorption curve is calculated.